Of all instruments shown on this web site this Leica TCRA 1101 robotic total station is the one that goes in the field most often. At the end of the 20^th century this instrument was the best total station money could buy (in the meanwhile Leica has produced two newer generations). It is still used on a weekly basis and one of the finest total stations built in the 20^th century.

The term "robotic" means that all controls are servo driven and that the instrument can rotate along its axis by itself (see figure 4). The TCRA1101 was not the first robotic instrument. By the end of the 1970s the French Minilir was already capable of driving its own axis to follow a target. After the Minilir several other similar instruments saw the market, like the German PolarTrack. When compared to those systems the TCRA1101 had several mayor advantages:

• Unlimited rotation along the vertical axis (both the Minilir and PolarTrack had a maximum horizontal range);
• Operation on internal batteries;
• Reduction of weight;
• Reduction of price;

The TCRA1101 was the first true robotic total station (i.e. that it could be used as an electronic theodolite). The advantage of the servo driven total station is that there is no end to the drive screw as with non servo driven instruments and that during the survey the instrument can point by itself to targets that need to be checked from a previous station, eliminating administrative errors that were common in the pre-robotic era. In addition to that these robotic instruments allow one-man-surveys where the instrument will automatically follow the target that is held by the man that also controls the instrument by remote control.

In addition to that the instrument has several useful features:

• Coaxial laser pointer;
• Reflectorless distances measurements;
• Extreme accurate digital circular vial;
• Laser plummet;
• PCMCIA/SD-card Data storage;
• RS232 Interface for remote control and automatic data collection (see figure 6).

According to the manufacturer the instrument has an accuracy of 1.5 arc seconds (0.5 mgon) for both circles. The vertical is determined by a liquid compensator, just like in the 1961 Wild T1A, although the technical implementation is differs completely. The accuracy of this vial is 0.5 arc seconds (0.2mgon) with a 4 arc minute (70mgon) working range. If necessary, the electronic vial can be switched off, allowing to use the instrument onboard of floating objects (see figure 7). The vial can be viewed using the onboard software which shows it as a circular vial in combination with digital output to the screen (see figure 5). The resolution of the digital on-screen vial is 2 arc seconds and 0.1 mgon.

In normal use the EDM measures a distance in 1 second with an accuracy of 0.002 metres up to a distance of 3.5 kilometres using a single Leica round prism. Using 3 prisms this can be extended to about 5.4 kilometres. In addition to that it can measure distances to reflection tape up to 250 metres, and without prism or reflection tape up to 80 metres. Using the extra power of the reflectorless mode distances up to and above 9 kilometres can be achieved when combined with 3 Leica prisms.

For centring the instrument is equipped with a laser plumb bob which is mounted in the primary axis and thus rotates with the instrument. The advantage of this set-up is that any deviation of the laser plumb can be detected by rotating the instrument 180 degrees around its primary axis.

Since long the sun has provided an accurate means to determine true north. In the 17^th century this was done by measuring the direction of the rising and setting sun using an azimuth compass. In more recent times theodolites and transits were used for the purpose, and even nowadays it is still taught to students using a total station (see figure 3). Demands for ever increasing accuracies required better methods to determine the proper direction of the sun.

In the second half of the 1940s Dutch professor R. Roelofs developed a new method of observing the direction of the sun using a theodolite. He designed a filter - which would become known as the Roelofs prism (see figure 10 and figure 11) - in which two optical wedges (see figure 12), set at right angles with each other, created four overlapping images of the sun. At the centre of these four overlapping suns a dark diamond shape remains that forms an easy target to aim at (see figure 13).

The first of these prisms was created by Dutch instrument manufacturer Van Leeuwen. Later the patent was taken over by Wild (model GSP1 - GSP3) and continued by Leica (GSP3). Initially the dark diamond had an angular offset from the sun's centre, but in 1953 a third wedge was added by which the diamond represented the actual centre of the sun.

Currently the production of Roelofs prisms is discontinued and the only alternative is a plain sun filter known as GVO13 (see figure 14). Although the GVO13 does not allow to directly observe the sun's centre (see figure 16 and figure 17), it still is a high tech piece of filter glass. According to their specs the filter has an optical density of 5 (a reduction to 0.000001)¹, but more importantly it is extremely flat (λ/100) and parallel (0.2 arc seconds)². Tests done with the specimen shown here revealed that it was indeed parallel within 0.1 arc seconds. As algorithms now exist to calculate the sun's azimuth with an accuracy of a few arc seconds, this parallelism ensures that the orientation of the filter on the objective has no significant effect on the observations.

Instead of directly measuring the centre of the sun the left and right limb are observed. I prefer to do this using the double vertical cross-hair of the reticle as this allows me to record three time stamps: one when it hits the left hair, one when it is halfway the two hairs and a third when it hits the right hair. This procedure is done twice: first when the right limb is observed, then when the left limb passes the vertical cross-hair.

Over the years the way the filters were delivered to the customers changed (see figure 15). The first Roelofs prisms by instrument manufacturer Van Leeuwen were delivered in a wooden box that was properly made of wooden planks. The prism by Wild was delivered in a wooden boxes that was made from a monoxylon, which was sawn in two halves after which a cavity was made in the two halves using a milling machine. The Leica prism had a artificial leather container with a zip. The GVO13, which is not much cheaper than the Roelofs prism, only comes in a plastic bag with some protective paper.